A numerical control unit is usually interfaced with a machine tool through a serial interface in accordance with an increased number of signals.
An example of this is shown in FIG. 3, wherein 1 designates the numerical control unit, and 70 designates an I/O base unit. Usually a plurality of I/O base units are used for a single numerical control unit 1. Two I/O base units 70, 80 are shown in the drawing, and since the I/O base unit 70 and the I/O base unit 80 have substantially the same arrangement, only the I/O base unit 70 will be described. The I/O base unit 70 is connected to the numerical control unit 1 through a serial interface drive unit such as relays and the like by a driver 72a. A serial signal from computer numerical controller (CNC) 1 is converted into a parallel signal by a serial/parallel converter 71. A signal from switches of the machine tool is received by a receiver 73a and the like, converted into a serial signal by the serial/parallel converter 71, and applied to the numerical control unit 1.
The I/O base unit 70 and the like are disadvantageous, in that they include a considerable number of I/O units, long parallel leads must be used to connect the I/O base unit 70 to the relays, the switches and the like of the machine tool and further, problems arise in the manufacture and cost thereof. Although the base unit 70 may be installed in the vicinity of the relays and the like, this causes another problem in that the parallel signal cables to the other switches and the like must be made longer.